The present invention relates to the field of test and measurement systems, and more particularly to an input protection circuit for a current shunt or for any analog measuring circuit including an overload monitoring scheme.
Scientists and engineers often use test and measurement and data acquisition systems to perform a variety of functions, including laboratory research, process monitoring and control, data logging, analytical chemistry, test and analysis of physical phenomena and control of mechanical or electrical machinery, to name a few examples. One example of hardware to implement such measuring systems is a computer-based measurement system or data acquisition (DAQ) system. A computer-based measurement or DAQ system typically includes transducers for measuring and providing electrical signals, signal conditioning hardware which may perform amplification, isolation and/or filtering, and measurement or DAQ hardware for receiving digital and analog signals and providing them to a processing system, such as a personal computer. The computer may further include analysis hardware and software for analyzing and appropriately displaying the measured data.
As mentioned above, a measurement system may include one or more of a measurement or DAQ device comprised in or connected to a computer system. The device may be an I/O board plugged into one of the I/O slots of the processing or computer system. The measurement or DAQ device may also comprise an external device connected to a computer system. Exemplary hardware I/O (input/output) interface include the GPIB (general purpose interface bus), the VXI bus, the PXI bus, or a serial bus such as the RS-232 protocol, IEEE 1394, or USB.
Measurement systems typically include circuits that are designed to protect the measurement system from analog voltage spikes, high voltages, and other harmful signals. Many such circuits comprise a fuse which protects the system from an excess amount of current. For example, a fuse may blow when the current rating of the fuse wiring is exceeded. However, fuses may be inconvenient or difficult to replace, such as on a board that is inside a computer. Hence a disadvantage of fuses relates to their inconvenience and/or impracticability in circuit boards in computers.
Furthermore, many measurement systems comprise current shunts which are used to measure current. A current shunt is a resistor in series with the load that is able to measure the current to the load. Current shunts may be protected by fuses; however, there exists the same disadvantage of being inconvenient or difficult to replace on a circuit board inside a computer.
Measurement systems may instead include a pair of transistors, such as MOSFETs, with a resistor in series between them. The MOSFETs and resistor act to protect the measurement system from an excess amount of current. The transistors are activated unless a voltage is applied to deactivate them. If there is no current flowing, then there is no voltage across the resistor and hence no voltage across the gates of the transistors. As current starts to flow through, voltage builds up across the resistor which then starts to deplete one or the other of the transistors. The circuit does not deactivate rapidly but instead forms a current source. The current source is determined by the resistor and threshold voltages of the transistors. The circuit is good for high frequency voltage inputs but the resistance is too high for current shunts. Another disadvantage is that the constant current source leads to excess power dissipation.
It would therefore be desirable to develop a circuit that provides input protection for a current shunt or for any analog measuring circuit including an overload monitoring scheme that does not implement fuses.
One embodiment of the invention comprises a circuit that provides input protection for a current shunt or for an analog measuring circuit which includes an overload monitoring scheme.
A measurement system may be configured to include an input protection circuit that protects a current shunt or any analog measuring circuit without the implementation of fuses. A plurality of transistors may be used to function in a similar manner to a fuse.
A particular embodiment of the input protection circuit may comprise the following: an input, a first transistor coupled to the input, a second transistor in series with the first transistor, a first detection circuitry coupled to the first transistor, a second detection circuitry coupled to the second transistor, and a current shunt in series with the second transistor. If the input exceeds a negative voltage threshold, then the first detection circuitry deactivates the first transistor. If the input exceeds a positive voltage threshold, then the second detection circuitry deactivates the second transistor.
Thus, one of the transistors may be deactivated once the input voltage exceeds a negative threshold voltage. The other transistor may be deactivated once the input voltage exceeds a positive threshold voltage. By deactivating the transistors once the input voltage exceeds a negative or a positive threshold voltage, the transistors function in a similar manner to a fuse and provide input protection for a current shunt or for any analog measuring circuit.
Another particular embodiment comprises a circuit providing protection for a first and a second current shunt, where the second current shunt is less in resistance than the first current shunt. The circuit comprises the following: an input, a first transistor coupled to the input, a second transistor in series with the first transistor and coupled to a switch to determine which current shunt is connected to the circuit, a voltage limiter coupled to the first and second transistor and to the input where the voltage limiter operates to selectively deactivate one of the first transistor or the second transistor if the input signal exceeds a first range, and a current limiter coupled to the first and second transistor and to the plurality of current shunts where the current limiter operates to selectively deactivate one of the first transistor or the second transistor if the input signal exceeds a second range. The second range is smaller than the first range.
If the input exceeds a negative voltage threshold of the first range, then the voltage limiter deactivates the first transistor. If the input exceeds a positive voltage threshold of the first range, then the voltage limiter deactivates the second transistor. If the input exceeds a negative voltage threshold of the second range, then the current limiter limits the current flowing through the first transistor. If the input exceeds a positive voltage threshold of the second range, then the current limiter limits the current flowing through the second transistor. Thus, the current limiter operates to protect the second current shunt and the voltage limiter operates to protect the current limiter.